The present invention relates to a solid golf ball, particularly a solid golf ball having a solid core, such as a two-piece golf ball or a multi-piece golf ball having three or more pieces.
The hardness (deflection) of the core material is known to play a large role in the feel of a solid golf ball on impact. By controlling the hardness of the solid core, it is possible to achieve the desired feel on impact in golf balls manufactured from existing materials. Recently, there appear to be quite a few golf balls having a plurality of intermediate layers and cover layers. By controlling the hardnesses of these respective layers, the abrasions that tend to arise between one layer and another (where they mutually adhere) can be reduced.
To manufacture golf balls of excellent durability without compromising the coefficient of restitution or the feel on impact, JP No. 2652502 describes the inclusion of a vulcanized rubber powder in a golf ball having a one-layer construction or in the core of a golf ball having a construction of two or more layers that includes a cover and a core of one or more layer.
However, one cannot expect to achieve a higher hardness (lower deflection) in a crosslinked rubber body by thus “increasing the durability without compromising the feel on impact.” This suggests that the ball hardness itself inevitably ends up being suppressed, with the ball having too soft a feel and not achieving the desired initial velocity, resulting in a golf ball that lacks a sufficient distance.
JP-A 2002-102388 describes a way of holding down the resilience by grinding up the cores of discarded golf balls, having the resulting material take up moisture, then adding the moisture-bearing material to golf ball cores.
However, when this method is used to hold down the resilience, the moisture inhibits the formation of a crosslinked structure in the rubber, which suggests that the desired hardness and initial velocity may be unattainable. Moreover, when the golf ball core itself has absorbed moisture, the properties of the rubber tend to be lost, which may also be a durability-lowering factor.
In addition, golf balls obtained by compounding substances such as rubber powder, cork, wood chips, powdered rice chaff in order to reduce the environmental impact from the reuse of waste materials are described in, for example, JP-A 2002-65895, JP-A 2001-25515, JP-A 2002-204837, JP-A 2002-302570, JP-A 2002-219192, JP-A 2004-49700, JP-A 63-212378, JP-A 61-94666 and JP-A 60-194968.
However, such prior art is not conductive to obtaining golf balls having an excellent durability, a core hardness that can be controlled with minimal decline in the initial velocity, and an excellent feel on impact.